Cut-off low

The cut-off low is a cold low (depression) in mid-latitudes (occasionally almost in subtropical latitudes) where air of polar origin is cut off from the main subpolar belt of low pressure and cold air, the normal track of depressions.

A cut-off low usually begins as a trough in the upper-air flow,
which becomes a closed circulation and then extends down to the
surface.
Also known as closed low or cold pool. A cut-off low is marked by more or
less concentric isotherms around the core of the low.

Cut-off lows are most frequent in periods of low index circulation . Sometimes a cut-off low occurs with a cut-off high over the higher latitudes, typically in blocking situations. Such slow-moving lows are associated with unsettled weather and, in summer, with thunderstorms.